This project involves the clinical investigation of children with MSUD and of their parents. We seek answers to the following questions: 1) Do obligate heterozygotes for MSUD manifest of biochemical defect in the disposal of leucine? 2) Would the acute administration of growth hormone be therapeutically efficacious in youngsters with MSUD who have suffered a metabolic relapse? 3) Do diet and plasma branched-chain amino acid concentrations affect performance on psychometric testing in children with.